Marshall low watt "Plexi" modifications

[Phrak9]

Hello,
I built this amp from a kit over 20 years ago and thought that it would make a nice teaching tool for me to grasp how mods to the tone stack change the character of the amp. I plan to build a 18 watt TMB variant in the near future.

It uses a 6005 power tube. I have questions how one would go about checking the tube bias? What component would be involved and how would I take a measurement? It has a hammond 262E6 PT 120v @ 87ma and 125A OT.

Also, I would like to determine how to set up the TSC with component values that are in the schematic. I think I have most of them entered correctly except for RIN and RL values.
Any suggestions, comments or advice are welcome.

tsc-1.png

Schematic.png

Circuit-1.png

Circuit-2.png

Circuit-3.png

Parts-1.png

[juanchankane]

Hi there, nice build. About biasing the 6005, it is similar to a 6v6, but in 7 pin package and with lower plate voltage. The 1ohm resistor at its cathode will make it easier for you to measure current through the tube. To check bias in a single ended, go and che Uncle Doug YT video. Once you get that right, then focus on the preamp, I'd bypass the tone stack (lift its ground) first, to see what amp voicing you are dealing with, adjust/tweak, and then focus on tone stack. Good luck and be safe with the high voltage!

[Crabman]

For calculating optimal bias voltage you will need the data sheet for your 6005. That will have the appropriate data plus graph with plate characteristic curves. You may need to draw the maximum dissipation curve for the 6005 onto that graph (using P=VI). You will need your plate voltage and slope of your load line which is determined by the impedance of your OT.

Check out the ValveWizard website and click on the single ended tab. Merlin shows how to use this informational to bias a single ended stage properly. Once you have the load line drawn you will be able to estimate the bias voltage and current at idle. You then use these parameters to calculate the required value of your cathode resistor (using R=V/I).

[Phrak9]

Very helpful, both of you. I will follow your suggestions and will likely have a few more questions as I go through it. Thanks again!

[Phrak9]

From the Uncle Doug video measuring voltage drop across the cathode bias resistor the wattage calculates to 1.95 watts. The OT is connected to the lug for 4 ohm loads.

Does this look like the measurement was done correctly?

Bias resistor 620 ohm (measured 617 ohm)
Plate voltage 302 V
Voltage drop 19.6 V
19.6/617 = .0649 = 64.9 ma
PV * PCma = watts
302v * 64.9ma = 19599.8 (1.95 watts?)

Location of PV measurement

2025-02-15_08-55.png

Not sure about the correct graph that is used, there's a few.

[JMPGuitars]

First question: Why is VAC written under the B+ nodes? B+ is rectified DC voltage, you should be measuring DC there.

Second question: What is the idle grid voltage?

Thanks,
Josh

[Crabman]

You made a mistake in your current calculation:

19.6V/617 ohm = 31.8 mA

So at 300 V the maximum dissipation for a 12 W tube is 40 mA. You are sitting at 80% of this dissipation which is fine. Perhaps a bit less since some of this is screen current.

You could go a bit hotter if you wish, but for that you need to plot the load line and get the correct grid curves plotted. The data sheet only plots up to a 250 V (max) anode voltage. You are above this value which is probably ok since these limits are often abused. The key is to make sure your load line doesn’t pass above the max dissipation curve.

So your curves for bias voltages are not correct since you are at 300v. If you look at the mazda belvu spec sheet for the 6005 you will see a transfer characteristics graph and you can estimate grid curves at 300 V from this. I can help with that if you wish.

if you look at the ValveWizard site .. push pull tab .. second chart down you will see that Merlin had plotted out a class A 4K load line for an EL84. This tube has the same max dissipation curve (12 W) that you have and he also plotted a bias point at 300V. Your impedance is also 4K and your bias is at 30 mA which is what he plotted also. So all of this applies to you except for the grid curves values which are not correct (since his is for an EL84). However, you can see how far up you can push this load line (not far) from 30 mA before you cross the 12W max dissipation curve.

[Phrak9]

Yikes! embarrassing. I will go over the method I used to get my erroneous numbers. And check the charts on valve wizard and try to get a handle on what I'm doing.

[Crabman]

No worries. Your measurements are good.

Looking at previous posts I misunderstood your OT info. You mentioned that you had it connected to a lug for 4 ohm loads .. must be the secondary side. I misunderstood that to be 4K reflected at the primaries. You need to figure out what is reflected at the primaries then you can plot things out properly. There should be multiple numbered tabs on the secondary side of that OT that you connect to. Looks as though you will have 2 wires going there. What are the numbers of the tabs where the wires are connected? Assuming you have it connected to a 4 ohm speaker we can determine the impedance reflected at the primaries and then plot out your load line.

I’m a bit confused by the OT. A hammond 125A is listed as a push pull type, but you have a single ended amp? Maybe i'm missing something.